1. Technical Field
This invention applies to gas turbine engines in general, and to methods for manufacturing air foils for use in gas turbine engines in particular.
2. Background Information
Airfoils disposed aft of a rotor section within a gas turbine engine help direct the gas displaced by the rotor section in a direction chosen to optimize the work done by the rotor section. These airfoils, commonly referred to as "guide vanes", are radially disposed between a hub and an outer casing, spaced around the circumference of the rotor section. Historically, guide vanes were fabricated from conventional aluminum as solid airfoils. The solid cross-section provided the guide vane with the stiffness required to accommodate the loading caused by the impinging gas and the ability to withstand an impact from a foreign object.
"Gas path loading" is a term of art used to describe the forces applied to the airfoils by the gas flow impinging on the guide vanes. The magnitudes and the frequencies of the loading forces vary depending upon the application and the thrust produced by the engine. If the frequencies of the forces coincide with one or more natural frequencies of the guide vane (i.e., a frequency of a bending mode of deformation and/or a frequency of a torsional mode of deformation), the forces could excite the guide vane into an undesirable vibratory response.
A significant disadvantage of conventional guide vanes made from solid aluminum is the cumulative weight of the guide vanes. Gas turbine design places a premium on minimizing the weight of engine components because increasing the weight of an engine negatively affects the engine's thrust to weight ratio. Hollow guide vanes made from conventional aluminum avoid the weight problem of the solid guide vanes, but lack the stiffness aid fatigue strength necessary for high thrust applications. This limitation is particularly problematic in modern gas turbine engines where the trend has been to increase the fan diameter of the engine to produce additional thrust. Increasing the thrust of an engine generally increases the loading on the guide vanes, particularly those in the fan section when the fan diameter is increased. An additional problem with hollow guide vanes made of conventional aluminum is that some of the more desirable conventional aluminum alloys cannot be extruded into the cross-sectional geometry required of a guide vane.
More recently, guide vanes have been produced from polymer matrix composite materials, or "PMC's". PMC's are attractive because they are significantly lighter than conventional aluminums, possess the requisite stiffness, and can be formed into a variety of complex geometries. A disadvantage of PMC guide vanes is the cost of producing them, which is significantly more than that of similar guide vanes made from conventional aluminum. Like weight, cost is of paramount importance. Another disadvantage of PMC guide vanes is their durability. Conventional aluminum guide vanes have an appreciable advantage in average life cycle duration over PMC guide vanes. Shorter life cycles not only require greater maintenance, but also exacerbate the difference in cost between the two materials.
In short, what is needed is a guide vane that possesses adequate stiffness and fatigue strength to accommodate loadings present in high thrust engines, one that possesses adequate stiffness and fatigue to accommodate foreign object strikes, one that is lightweight, one that is relatively inexpensive to manufacture, and one that can be readily manufactured.